We are conducting investigations aimed at elucidating the mechanisms involved in the genetic regulation of cell differentiation. We are pursuing these studies using Drosophila melanogaster because it is one of the most suited organisms for investigating differentiation using biochemical and genetic tools. Specifically we are studying the regulation of kynurenine hydroxylase which is an enzyme in the pathway which converts tryptophan to the eye pigment xanthommatin. This enzyme has several biological properties that makes it one of the most favorable enzymes with which to study gene regulation during development. These are, bimodal distribution during development and limited tissue distribution. It is a non essential enzyme and therefore genetic manipulation can be accomplished without lethality. In addition it produces a phenotype, mutations of which are easy to observe. These properties have led us to propose here a selection scheme for obtaining mutations in genes which have a regulatory function with respect to kynurenine hydroxylase. This enzyme also has the unique property of appearing in a specific imaginal disc, the eye disc, as it differentiates. By purifying this enzyme we will then be able to apply immunochemical procedures to measure synthesis of this specific gene product in whole animals or in culture of mass isolated discs and not have to purify and culture a large number of specific imaginal discs. This is not yet possible. Imaginal discs have been recognized as a potentially powerful system with which to investigate differentiation.